1. Field of the Invention
The present invention relates to a thermosensitive stencil paper for printing, and more particularly to a thermosensitive stencil paper capable of producing high quality images, without causing the problem of sticking to a thermal head during the preparation of printing masters.
2. Discussion of Background
Conventionally, many patent applications directed to the prevention of the sticking of thermosensitive stencil papers to a thermal head during the preparation of printing masters have been filed. More specifically, these thermosensitive stencil papers are prepared by laminating a thermoplastic film such as a crystallized polyester film with a thickness of about 2 .mu.m and a melting point of 245.degree. to 260.degree. C. on a porous substrate, and a variety of overcoat layers comprising a sticking-prevention agent on the polyester film.
For example, the following overcoat layers have been proposed:
(1) an overcoat layer comprising a metal salt of a fatty acid (Japanese Laid-Open Patent Application 60-19592);
(2) an overcoat layer comprising a phosphate surface active agent (Japanese Laid-Open Patent Applications 61-102294, 61-102295, 61-114893, 61-114894 and 61-125897);
(3) an overcoat layer comprising a room-temperature-curing silicone varnish (Japanese Laid-Open Patent Application 58-153697); and
(4) an overcoat layer comprising an ultraviolet-curing silicone varnish (Japanese Laid-Open Patent Application 61-295098).
Even when any of the above-mentioned overcoat layers comprising the sticking-prevention agent is provided, however, the effect of preventing the sticking problem deteriorates with time. In particular, the sticking problem often occurs when the overcoat layer is melted corresponding to a solid image formed on an original under application of the thermal energy from a thermal head to the overcoat layer. The overcoat layer tends to stick to the thermal head, and as a result, the thermoplastic film is peeled from the porous substrate. In addition, when part of the thermoplastic film sticks to the thermal head, the frictional resistance between the stencil paper and the surface of the thermal head is increased, so that the stencil paper cannot be smoothly transported. Consequently, the image obtained on the stencil paper substantially shrinks in comparison with the image of the original. Thus, the dimensional reproduction performance of the image formed on the printing master is degraded. Furthermore, the thermosensitive stencil paper becomes creased when transported on a platen roller for printing operation.
Furthermore, as disclosed in Japanese Laid-Open Patent Application 62-282983, a high-sensitivity film is proposed to prepare a printing master at high speed with low energy. This type of film, which is made of a polyester resin or polyamide resin, has a thickness of 5 .mu.m or more and is singly used as a stencil without attaching it to a porous substrate.
Such a high-sensitivity film tends to readily cause the sticking problem as compared with the aforementioned thermosensitive stencil paper comprising the crystallized polyester film when used to make a printing master. To solve the sticking problem, therefore, there are conventionally proposed high-sensitivity films comprising as a sticking-prevention agent a variety of materials, such as an amino-modified silicone oil (Japanese Laid-Open Patent Application 1-238992), an ester-modified silicone oil (Japanese Laid-Open Patent Application 1-237196), and a mercapto-modified silicone oil (Japanese Laid-Open Patent Application 2-89694). However, the effect of preventing the sticking problem by us of these sticking-prevention agents deteriorates with time and these conventional sticking-prevention agents are unsatisfactory for use in practice.